Semiconductor modules are widely used in power conversion apparatus represented by hybrid vehicles, electric vehicles, and the like. Semiconductor modules included in such energy-saving controllers include power semiconductor elements for controlling a large current. Normal power semiconductor elements generate heat at the time of controlling a large current. As power conversion apparatus are downsized or their output becomes higher, the amount of heat generated increases. Accordingly, it matters very much how to cool semiconductor modules including a plurality of power semiconductor elements.
In order to improve the efficiency of cooling semiconductor modules, liquid cooling coolers have traditionally been used. Various devices are adopted in order to improve the cooling efficiency of liquid cooling coolers. For example, a coolant flow rate is increased, the shape of a heat radiation fin (cooling body) is determined so that a high heat transfer coefficient will be obtained, and the thermal conductivity of a material for a heat radiation fin is raised.
However, if the flow rate of a coolant flowing to a cooler is increased or if a heat radiation fin is shaped complexly to give it a high heat transfer coefficient, then a load on a cooling pump for circulating the coolant increases. For example, a loss in the pressure of the coolant increases in the cooler. With a cooler in particular which cools many power semiconductor elements by the use of a plurality of heat sinks and in which a plurality of flow paths are connected in series, a significant increase in pressure loss takes place. Ideally, cooling efficiency is improved by a low coolant flow rate in order to reduce pressure loss. For example, the thermal conductivity of a material for a heat radiation fin may be improved. However, the adoption of a material for a heat radiation fin having high thermal conductivity may lead to an increase in the costs of an entire cooler.
In order to reduce pressure loss while maintaining cooling performance, formerly coolers in which a coolant introduction flow path for introducing a coolant and a coolant discharge flow path for discharging the coolant are arranged in parallel with each other and in which a plurality of heat sinks are arranged between the coolant introduction flow path and the coolant discharge flow path in a direction approximately perpendicular to the coolant introduction flow path and the coolant discharge flow path were proposed (see Japanese Laid-open Patent Publication No. 2001-35981 (paragraph no. [0020] and FIG. 1), Japanese Laid-open Patent Publication No. 2007-12722 (paragraph no. [0006] and FIG. 7), Japanese Laid-open Patent Publication No. 2008-205371 (paragraph no. [0021] and FIG. 1), Japanese Laid-open Patent Publication No. 2008-251932 (paragraph nos. [0037] and [0038] and FIG. 7), Japanese Laid-open Patent Publication No. 2006-80211 (paragraph no. [0006] and FIG. 1), Japanese Laid-open Patent Publication No. 2009-231677 (paragraph nos. [0024] and [0031] and FIG. 2), Japanese Laid-open Patent Publication No. 2006-295178 (paragraph nos. [0017] to [0024] and FIG. 2), and Japanese Laid-open Patent Publication No. 2010-203694 (paragraph no. [0026] and FIG. 3). In that case, a coolant flows in parallel between fins included in a heat sink, so cooling performance can be improved. In addition, a loss in the pressure of the coolant in a flow path can be reduced (see Japanese Laid-open Patent Publication No. 2006-80211).
Furthermore, a liquid cooling cooler in which flow paths (header water routes 11a and 11b) for introducing and discharging cooling liquid are arranged on the same side of a module and in which each flow path is arranged in a direction perpendicular to fins with no change in cross-sectional area is proposed (see, for example, FIG. 1 in Japanese Laid-open Patent Publication No. 2008-205371). As a result, a loss in the pressure of the cooling liquid can be reduced to the utmost.
In addition, a liquid cooling cooler in which the whole of a rear-side wall of a casing that is a cooling liquid inflow section smoothly inclines to a front side from a right-side wall side toward a left-side wall side and in which the cross-sectional area of a flow path in an inlet header portion becomes smaller from a cooling liquid inlet side toward the left-side wall side is proposed (see, for example, Japanese Laid-open Patent Publication No. 2009-231677). In this case, the distribution of flow speed in all flow paths in a parallel flow path section of the casing, that is to say, the distribution of flow speed in the direction of the width of the parallel flow path section becomes uniform.
In a semiconductor module cooler there is clearance as space between a fin, which is a heat sink, and a bottom of the cooler. In particular, however, if the clearance is more than an interval between adjacent fins, then a coolant flows to the clearance and does not flow sufficiently between fins. Accordingly, the clearance is narrowed. However, if a dimensional tolerance at the time of the assembly of parts is taken into consideration, too little clearance is not desirable. Even if there is much clearance, it is important to enhance a cooling effect by a coolant.
With conventional cooling techniques, on the other hand, the shape of a heat sink or a coolant flow path, a method for arranging elements which generate heat, the shape of a coolant introduction inlet or a coolant discharge outlet, or the like causes non-uniform distribution of coolant flow speed in a cooler. Such non-uniform distribution of coolant flow speed leads to non-uniform cooling performance. Accordingly, with conventional coolers it is difficult to obtain uniform and stable cooling performance. In addition, troubles, such as a significant rise only in the temperature of a semiconductor element arranged diagonally opposite to a coolant discharge outlet, arise. As a result, the lifetime of the semiconductor element becomes short or a failure or the like tends to occur.
Furthermore, with the above cooler (see, for example, Japanese Laid-open Patent Publication No. 2009-231677 or No. 2006-295178), the cross-sectional area of a flow path in an inlet header portion becomes smaller in a direction in which the flow path extends. Accordingly, there is a tendency for flow rate distribution to improve. However, a rise in temperature near a coolant introduction inlet is not solved. Even if only flow speed adjustment is made by changing the shape of an introduction flow path, pressure loss tends to increase.
By the way, with the above liquid cooling cooler (see, for example, Japanese Laid-open Patent Publication No. 2010-203694), a plurality of flow path groups each of which includes a plurality of flow paths and which differ in path resistance are placed side by side in the direction of the width of a parallel flow path section. This makes it possible to make flow speed distribution in the direction of the width of the parallel flow path section uniform. This prevents the appearance of a portion in which cooling performance falls off due to a decrease in flow speed. Because of the influence of, for example, a warp of a fin base which occurs in a process for manufacturing a cooler, however, it is not easy to obtain stable cooling performance.